1. Field of the Invention
The present invention relates to solenoid valves including a solenoid part that moves a spool valve in the axial direction with respect to a tubular sleeve having a supply port and an output port.
2. Description of the Related Art
Conventionally, a solenoid valve including a tubular sleeve, a spool valve, and a solenoid part is used to control, e.g., a hydraulic device of an automatic transmission of a vehicle (see, e.g., Japanese Patent Application Publication No. 2014-185748 (JP 2014-185748 A)). The tubular sleeve has a supply port to which hydraulic oil is supplied and an output port that outputs hydraulic oil. The spool valve moves in the axial direction in a valve hole formed in the sleeve to change the area of a flow path between the supply port and the output port. The solenoid part operates when an exciting current is supplied thereto, and presses the spool valve toward one side in the axial direction.
The solenoid part of the solenoid valve described in JP 2014-185748 A has an electromagnetic coil, a solenoid core, a plunger, a shaft, and an annular stopper. The solenoid core serves as a magnetic path for magnetic flux generated by the electromagnetic coil. The plunger moves in the axial direction with respect to the solenoid core. The shaft moves with the plunger in the axial direction. The annular stopper is fitted on the shaft. When an exciting current is supplied to the electromagnetic coil, the plunger moves in the axial direction with respect to the solenoid core, so that the shaft presses the spool valve.
FIG. 6A is a configuration diagram showing an example of the configuration of a solenoid part of a conventional solenoid valve 9. FIG. 6B is a partial enlarged view of FIG. 6A. In FIG. 6A, the portion above an axis O shows the solenoid valve 9 in a non-operating state (no exciting current being applied to an electromagnetic coil 90), and the portion below the axis O shows the solenoid valve 9 in an operating state with an exciting current having a rated current value being applied to the electromagnetic coil 90 of the solenoid valve 9.
As shown in FIGS. 6A and 6B, the solenoid part of the solenoid valve 9 is configured so that a plunger 92 moves in the axial direction with respect to a solenoid core 91 by the magnetic force of the electromagnetic coil 90. The electromagnetic coil 90, the solenoid core 91, and the plunger 92 are accommodated in a solenoid case 900.
The solenoid core 91 has a cylindrical part 911, an annular extended part 912, and an annular plate-shaped rib part 913. A shaft 93 that moves with the plunger 92 in the axial direction is inserted through the center of the cylindrical part 911. The extended part 912 is extended in the axial direction from an outer peripheral end of one end of the cylindrical part 911 toward the plunger 92. The rib part 913 projects outward from the other end of the cylindrical part 911. The extended part 912 is formed by a tapered part 914 and an annular extending part 915. The tapered part 914 is formed such that the outside diameter thereof decreases further away from the cylindrical part 911. The extending part 915 extends in the axial direction from a tip end of the tapered part 914.
As shown in the expanded view of FIG. 6B, an outer peripheral surface 914a of the tapered part 914 has a conical shape whose outside diameter decreases closer to the extending part 915. An outer peripheral surface 915a of the extending part 915 has a cylindrical shape whose outside diameter is the same as a part of the outer peripheral surface 914a of the tapered part 914, which has the smallest diameter. An inner peripheral surface 912a of the extended part 912 has the same bore diameter along the entire axial length of the tapered part 914 and the extending part 915 and faces an outer peripheral surface 92a of the plunger 92.
An annular stopper 94 is interposed between the cylindrical part 911 of the solenoid core 91 and the plunger 92. The stopper 94 is made of a nonmagnetic material and is fitted on the shaft 93. As the plunger 92 approaches the solenoid core 91 (the cylindrical part 911), the stopper 94 is sandwiched between the cylindrical part 911 of the solenoid core 91 and the plunger 92, whereby axial movement of the plunger 92 toward the solenoid core 91 is restricted. The stopper 94 prevents the plunger 92 from not being separated from the solenoid core 91 due to the residual magnetism in the solenoid core 91 and the plunger 92.
When an exciting current is supplied to the electromagnetic coil 90, the plunger 92 is moved toward the cylindrical part 911 with respect to the solenoid core 91 by the magnetic force of the electromagnetic coil 90. The shaft 93 moves with the plunger 92 to move a spool valve 95 with respect to a sleeve 96. If supply of the exciting current to the electromagnetic coil 90 is cut off, the spool valve 95 is moved toward the solenoid part by the restoring force of a spring, not shown, and the plunger 92 is separated from the cylindrical part 911 of the solenoid core 91.
In order to accurately control the solenoid valve 9 configured as described above, it is desirable that an axial force that is applied to the plunger 92 (attraction toward the solenoid core 91) vary only slightly even if the axial position of the plunger 92 with respect to the solenoid core 91 changes when a fixed exciting current is supplied to the electromagnetic coil 90. It is also desirable in terms of power saving and reduction in size of the solenoid valve that the plunger 92 be subjected to a larger axial force for the exciting current that is supplied to the electromagnetic coil 90. Typically, the axial force that is applied to the plunger 92 when a fixed exciting current is supplied to the electromagnetic coil 90 tends to increase in the central part of a movable range of the plunger 92 relative to the solenoid core 91, or a range in which the plunger 92 can move relative to the solenoid core 91, and tends to decrease at both ends of the movable range.
In order to increase the axial force that is applied to the plunger 92 when the plunger 92 is located farthest from the cylindrical part 911 of the solenoid core 91 in the solenoid valve 9, it is effective to increase the radial thickness of the extending part 915 of the extended part 912 and to increase the tilt angle θ of the outer peripheral surface 914a of the tapered part 914 with respect to the axial direction, as shown by phantom line (long dashed double-short dashed line) in FIG. 6B. However, changing the shape of the extended part 912 in this manner causes a disadvantageous phenomenon. Namely, when the plunger 92 approaches the cylindrical part 911 of the solenoid core 91, a radial component of the magnetic force that is applied to the plunger 92 increases and an axial component (the axial force described above) of this magnetic force decreases.